Arthroplasty is the standard of care for the treatment of shoulder joint arthritis. A typical humeral head replacement which attempts to mimic anatomic conditions involves a metallic humeral stem and a modular head replacement, capable of multiple positions for optimal boney coverage. Such humeral head replacement articulates with an opposing glenoid resurfacing device, which may be manufactured from UHMWPE.
For more severe cases of shoulder arthritis, the standard treatment is a reverse reconstruction, which includes reversing the kinematics of the shoulder joint. This is performed by securing a semi-spherical metallic device to the glenoid, referred to as a glenosphere, and implanting a metallic humeral stem with a modular cavity, typically manufactured from ultra high molecular weight polyethylene (UHMWPE), capable of receiving the glenosphere. The metallic humeral stem is usually offered in one fixed orientation that does not provide any eccentric adjustability to allow for proper fill in the humeral metaphysis.
As patient disease may progress after anatomic treatment, revision surgery may be necessary to perform a reverse reconstruction of the shoulder. Removal of anatomic devices that have integrated into the patient's boney anatomy proves to be difficult for the surgeon, and could potentially cause excessive patient bone loss.
Stems for shoulder prostheses typically permit a variety of neck angles to accommodate differing patient anatomies; such neck angles may range from 125 to 140 degrees, for example. Such stems are typically monoblock, which requires a separate stem implant component to be provided in a surgical kit for each neck angle, as well as each neck angle for each stem length option provided. This results in a large amount of inventory and thus design constraints on the number of angles that may be added to the implant range and/or provided by a single prosthesis surgical kit.
The anatomical neck of the humerus may be resected anywhere within a predetermined angle range, but the resultant angle is measured and the “closest” angle stem may be selected. This may result in minor non-conformities that are either not addressed (which may result in sub-optimal implant support and imaging) or corrected with secondary reaming (which may require an extra step and more time for the patient in the operating room). In addition, multiple monoblock stems provided in a surgical prosthesis may also require multiple trial implants to determine if the chosen neck angle is sufficient. Stem trialing often involves the insertion and removal of a part in the prepared humerus, which may potentially compromise the fit with the final implant in the humerus. This also adds an additional operating step, thereby increasing surgical time and exposure to anesthesia.